US2492720A

US2492720A - Hydraulic drive and speed control means therefor

Info

Publication number: US2492720A
Application number: US598822A
Authority: US
Inventors: Tyler Ransom
Original assignee: Oilgear Co
Current assignee: Oilgear Co
Priority date: 1945-06-11
Filing date: 1945-06-11
Publication date: 1949-12-27
Anticipated expiration: 1966-12-27

Description

Dec. 27, 1949 R. TYLER 2,492,720

' HYDRAULIC DRIVE AND SPEED CONTROL mums THEREFOR Filed June 11, 1945 LOAD INVENTOR Ransom TYLE ATTORNEY Patented Dec. 27, 1949 HYDRAULIC DRIVE AND SPEED CONTROL MEANS THEREFOR Ransom Tyler, Greenfield, Wis, assignor to' The Oll'ge'ar Company, Milwaukee; Wis., a corporation of Wisconsin ,Appiieaummnen, 1945; Serial No. 598,822

This invention relates to hydraulic drives or the type which includes a rotary hydraulic motor for driving amachine-or an element of a machine,

a pump for supplying liquid to themot'or toenergize it, and means'for controlling'thespeed of the machine.

Certain machines are operable through a wide range of speeds and require a high driving'torque at low speeds-but-require only a moderate torque at high speeds. Such amachine should be gradually acceleratedto a speed which may either be substantially constant 'orbe proportional to a reference speed.

The present invention has as an object tojprovide a hydraulic -drive* which *is particularly adapted for driving a machine-0f the above type."

Another object is to provide a hydraulic drive capable of exerting a high torque through one part'oi a speedrange and-maintaining a constantpower output through another'part of'the speed range.

Another object is to provide a hydraulic drive which will accelerate-a machine-or other driven element to and maintain it'ata speed which may either be substantially constant or be'pr'oportional to a reference speed.

Another object is toprovid'e a hydraulic drive of: the above character which is susceptible of close adjustment and control.

A drive constructedaccording to the invention has the advantage that its pump may be considerably smaller than the pump which would ordinarily be required.

Other objects and advantages will'be apparent from the following description of a drive embodying the invention and shown'schemat'ically in the accompanying drawing.

The drive chosen for illustration includes a pump l which is adapted-to bedri ven' at a--substantially constant. speed from a prime mover" or other power source 2,.am'ot'or 3' which-is ad'apted to be connected to a machine or other load "4' to drive the same, and twofluidchanjnelsi and 8 which connect pump I-and.motor:3 tOBflChLOthEI and form therewith a hydraulic circuit.

Pump l is ordinarily .mounted'upon .or connected to a suitablereservoir andprovided-wlth means for supplying liquid from the reservoir to the return side of-the circuit to-makeupleakage losses and to keep the circuit flooded.- Since such'pumps are well knownl'and inextensive 'use, pump I has not been illustrated in detail. It is deemed sufficient to. state: herein: tha-tpump ines a displacement varying member or slide block I which is shiftable between aeneutral or zero-dis! placement position and a maximum displacement position, and that pump I when driven will deliver liquid through channel 5 to motor 3 at a rate proportional to the distance that slide block I is ofi set from its neutral position.

Slide block 1' is ordinarily urged toward its neutral position by a substantially constant force. As shown,. slide block 7 is urged toward neutral by alow-constant pressure created by a gear pump 8 and-acting upon a piston 9 which engages slide block '1' and is fitted in a cylinder l0 carried by the pump casing.

The movement of slide block 1 toward the rightis limited in a suitable manner. As shown, piston in is adapted'to engage a stop ll fitted in the end; "wall of cylinder 10' and engaged by a :screw 12' which may be adjusted to stop slide block 1 from moving farther toward the right when .it'has reached any desired position. "the present instance, screw I2 is ordinarily ad- 'iustedto stopslide block I in its maximum displacement position;

'Gear pump' 8 is ordinarily driven in' unison with pumpland arranged within the casing thereof according tothe usual practice but, for the purpose of illustration, it has been shown separate from pump I and as drawing liquid from a reservoir' l1 and discharging it into a branched low pressure channel 18 one branch of which is connected'to cylinder I0.

Reservoir H is ordinarily the same reservoir from which pump I is supplied with liquid but for the purpose of illustration it has been shown separatefrom ump 1-.

-substantially constant low pressure equal to the resistance of relief valve I 9.

Slide block '[is adapted to be moved toward the right by liquid acting upon a piston 20 which engages slide block 1 and is fitted in a cylinder 2| carriedbythe. pump casing. Liquid for operating piston 20' is supplied to cylinder 2| througha channel 22 in response to operation of a control .mechanism as will presently be explained.

23 .is-positioned-by meansof a screw 24 which may be adjusted to cause stop 23 to prevent slide block Tirom moving rfarther toward the left when it reaches any desired position. In the present a added to the numerals applied to the parts of motor 3.

' That is, motor 3 has a displacement varying.

member or slide block "i a piston 9 fittedin 2.

cylinder Iii and adapted to move slide block 1 toward the left against a stop 23 fitted in the end of a cylinder 2| and adjusted by means of a screw 24 and a piston fitted in cylinder trol for varying its displacement. As shown, it is provided with a control stem 36 rotation of which changes pump displacement. Stem 36 may be rotated through a reduction gear by a small electric motor adapted to be controlled from a remote point in a manner which is well known as such motor controls are in extensive commercial use but, for the purpose of illustration, stem 36 has been shown as being adapted to be rotated by a handwheel 3! fixed thereto.

Liquid for keeping the hydraulic circuit of the control transmission flooded may be supplied ,from reservoir I! by gear pump 8 but the control transmission is ordinarily provided with its own 2 l and adapted to move slide block l toward the right against a stop l fitted in the end of cylinder Iii and adjusted by means of a screw l2.

An increase in displacement is effected by movement of slide block l toward the right the same as in pump l, but instead of slide block l being constantly urged toward the left by gear pump liquid and adapted to be moved toward the right in response to operation of the control mechanism as is pump slide block I, it is constantly urged toward the right and initially held in a maximum displacement position by gear pump liquid supplied through a branch of channel ii! to cylinder 2| and acting upon piston 26 and it is adapted to be moved toward the left to decrease motor displacement by liquid supplied by the control mechanism through channel 22 to cylinder Ill and acting upon piston 5 as will presently be explained.

The maximum movements of slide block I are determined by screws li and 24 which may be adjusted to stop slide block F in any two desired positions. In the present instance, screw I2 is ordinarily adjusted to stop slide block 1 from moving farther toward the right when it reaches a position at which motor displacement is maximum, and screw 2& is ordinarily adjusted to stop slide block F from moving toward the left beyond a minimum displacement position such as a position at which motor displacement is about one fourth of maximum.

Liquid for operating pistons Zll and 9 is supplied by a control transmission including a pilot pump 39. a pilot motor 3!, a channel 32 through which pump delivers liquid to motor 3| and a channel 33 through which liquid is returned from motor 3| to pump 30.

Pilot motor 3|, which may have means for varying its displacement but which has been shown as a constant displacement motor, is driven in unison with main motor 3 as by being driven from the main motor shaft through a suitable drive 34.

Pilot pump 3|] may be driven in unison with main pump l as by being connected by a suitable drive to the drive shaft of pump I or by being driven directly from power source 2. Since pump is ordinarily driven at a substantially constant speed, pump 30 when driven in unison with pump i would have a speed as nearly constant as is necessary for most commercial uses.

However, since motor 3 may be located a considerable distance from pump I and since the control transmission ordinarily employed is a commercial type in which the pump and the motor are arranged within a single casing, pilot pump 30 has been shown as being driven by an electric motor 35 which operates at a substantially constant speed.

Pilot pump 38 is preferably of the variable displacement type and provided with a suitable conreservoir, such as the reservoir 38, and provided with means for supplying pump 30 with liquid from" that 'rese'rvoir according to the usual practice.

The control transmission has its pressure channel 32connected by a channel 39 to channel 22 through a flow' restricting means such as an adjustable orifice choke 49 which limits the rate at which liquid mayfiow from the control transmission to channel 22 and thereby limits the rate at which the main pump displacement is increased and the rate at which the main motcr displacement is decreased;

In order'that main pump displacement may be rapidly decreased and main motor displacement may be rapidly increased, a check valve 4| is connected in parallel with choke 40 to permit liquid to flow freely from channel 22 to the control transmission and to prevent flow from the control transmission to channel 22 except through choke til. As shown, check valve 4| has its inlet connected to channel 22 by a channel 42 and its outlet connected to channel 39 by a channel 43.

The drive may be started and stopped by means of a 'valve which whenopen will short circuit pilot pump 30. As shown, a branch of channel 43 is connected to a valve casing 44 having a valve 45 fitted therein to control communication between channel 43 and a channel 46 which discharges into the reservoir from which pump 30 is supplied with liquid, such as reservoir 38.

, for operation from a remote point.

Operation With the parts in the positions shown and the ps un in pump 30 is bypassed through valve casing 44, as indicated by the small full arrows, and gear pump 8 is maintaining pressure in cylinder H1 and causing piston 9 to hold .pump I at zero displacement so that no liquid is discharged by pump and motor 3 is idle. Gear pump 8 is also maintaining pressure in cylinder 2| and causing piston 20 to hold motor 3 at maximum displacement.

The drive is started in operation by shifting valve 45 to stop the flow of liquid into exhaust channel 46 and then, since pilot motor 3| is idle so that liquid cannot enter it, the liquid discharged by pump 30 must flow, as indicated by the small dotted half arrows, through channel 39 and choke 40 into channel 22 at the rate determined by choke 40. If at this time pump 30 should discharge liquid at a rate in excess of the rate at'which liquid can flow through choke 4D, the excess liquid would be exhausted through a relief valve which, as is customary in power rwmaao pumps;isarranged with lnthezpmp haninz enid hence would i not app'ear in the 'drawing.

The liquid delivered 'thrmtg'h rchoke fl *tdehannel 22 creates therein aipressure which -aacts upon pistons 2 and 9 11 and terids tonrlovia them but piston 'M] hasa much larger eflective .epressure area than. piston :9 so thatlpistondfl will move .pump slide block '1towlard thediight from -its gear a pump relief valve 2 t9.

As :soon as slide block 1: moves from its neutral 7 position, pump 1 l willfistart -to discharge-"he um at a rate propor-tionabto-the distance that slide block I is-moved irom its-neutralzposition. 'Ihe channel to motor '3-a-nd :causeittodrive load 4 and to return liquid to; pumpi throughchannel 6, the flow of-liquid-i-n-the main-eircuitbeing indicated by large dotted arrews. sincelpiston 28 is holding slide blocl -1;.in i-ts maxi-mumdisplacement position,-motor3- iscapable-ef-exerting its maximum torque.

Pilot motor 3| will-start to cperate-in unison with motor 3 and -it-wi;ll transfer liquid from channel 32 to channel as indicated by the small dotted arrowsbut since it at -firstioperates at a very slow speed, all of-the liquid discharged by pilot pump 3D-cannot pass=through-it. Consequently, liquid will continue -DfQGW-llhIOI1h channel39, choke 4!] and -channelfltocylinder 2| and cause. piston togradx-ially increase'the displacement of pump! andtherby increase the rate at which pui-np l delivers-liquid-to motor- 3 which is thusgradually accelerated andngradually increasesthe speed of pilot -motor 3L N If the displacement of: pilotspump 1s relatively small and -.remains -unchanged, -the displacement of pump] :and the speed-cf motor 3 will be gradually i-ncreasedin'the abovemanner until motor-"3- is driving..pi lot-motor 'ifl just fast enough to enable it'to consume an of 'the liquid discharge'd'by pilot pump 3fl so'fthat there is no excess liquid "for .efiecting ffui'ther operation or isto zu. fThen thasped er motor '3 will remain substantially 'constai'itas'long as the speed of pilot pump '30 remains sabstsnuaiiy' constant.

If the dispiacementior-piloi.pump at "isiinitially large enough :or if it-isegraduallyincreased to a predetermined maximum after the drive .starts to operate, pump'3flwill continue to -deliver liquid faster than motor 3 banwgnsurnedt. Consequently, liquid will continuetoilowthrough channel 39, choke and channel 22 -toAcyIinderiZl and cause piston 20 --to .gradually;;increa se the displacement of a pump Large therebyi gradually increase the speedof motOr B-until the displacement of pump I--becomes maximum. L-Motor 3 is capable of exerting its maximum torqueduring this part of its speed range ior theareason that its displacement remains maximum.

When the displacement ofhpump 1 becomes maxi-mum; piston S engagesstop-l l; and prevents further adjustment-oi pump l.. Then thepr-essure created bypump; 30 =w;ill-- riseandthe-liquid delivered to channel '22 will enter cylinder l0 and cause piston 9 to shiftrslide block l -toward the left to gradually decreaseethe displacement of motor 3.

When a motor issupplied withliquid at aco nstant rate andam a constant pressure decreasing motor displacement-manure. correspending-iin- '6 crease? inwmotor ispeed andra corresponding Idecrease in imotor torque. Th'ereiiore, eg-raduall y 'shiftingslide block'l toward the leit'ca use's motor 3 to gradually accelerate-and to: exert \acgradually rlecreasi-ng torque iso :that r its-power :output "remains-substantially 'constantwafter slide I block "1 starts to move.

Motor 3'-will gradually accel-erate until 'it is driving pilot? motor 3 [just iast:zeriortgla to. enable wit to consume all of thee-liquid discharged by pilot pump 30 so that -theredsnoiexcess-liquid for effectingfurther operation of piston 9 Then the :speed 50f motor- 3 will remain :substantially constant as;rlong=:asthe speedfianddisplacement of.pi-l-ot;pump ,3ll remains substantially constant. Aaf-ter the drive has: been :broughtuptospeed, a it may be :deceleratedl'bydecreasingthe=speedor the displacement ofpil'ot umpdfl: and thenpump 3fl -willnot discharge. enough liquid -to suppiy pilot motor -3 I. 'Enou'g-h liquid 'tmmake up the difiere'nce between the requirements oi motorfil and the cut-put of 1 pump :39 will thenbedrawn -by .:motor 34 --throu'ghchannels-:32, 39 rand dfi, check waive-'41 "andtcha'nnels -42 and 22 from cylinder 2| if the drive is then operatingiin the lower partvof its speed range: ;:or from; cylinder In if the drive is' then'operati-ng in -the upper :part of its'speed range. V p

1. Drawing liquid from cylinder '2] permits: gear .pump pressure acting uponl piston 91 .to :move slide -.-block .1 toward the left-to reduce the-displacement a of pump I andthereby decrease-the speed otmotor= 3. Drawing liquid: .iromcylinderym apermits gear pump pressure asting upon piston 26 to move slide block l towardrtheright to increase the :displacement of motor 3- an d-thereby decrease the speed of motor 3.

-Pilot: motor a 31 will decelera-te in unison with m'otor :3 until .its volumetric intake is 1 just equal to the volumetric output of pump 30 so that no more liquid can: escape from. cylinder 2! -or from x'cylir'icler gt-6?. Then the speed -'of motor ll-will 'remain substantially .constant as long las the speedand the-displacementef pilot pump-30 re- -mains substantially constant. I

If after the drive has been brought-up-to such Y. a: speed that pilot motor-3 l a. consumes liquid. just as fast but no faster thanapilot pump 3illdischarges1-liquid,--the speed ofmain motor 3 will be maintained proportional-tothe speed-of. pilot pump --3 0 regardless OiW-hethenpump :Lis'dr'iven ate constant speed or at-a variable:speed.

- For example, if the speed 61? motor 3 should tend to decrease or ii the speed of pump 33 should 1 be increased,pilot-motor -3! will not be driven v fast enough to A consume all or the"liquid discharged by pump 30 and the excess liquid would operate the displacement varying mechanism, as explained .above,ito cause motorsfiand -3l to accelerate untilthewblu metrifc intake of .pilot motor T31 exactly equalldthe' volumetric output of pilot pump 39. Conversely, if thet'speed 'of motor-3 should 'increa'seor ifthe speedof pump 30 should decrease, motor '3] would con-- sumemore liquid than was discharged by pump 30 and the deficiency would be made up byliquid -drawn from'cy'linderil or cylinder Ill so'that the displacement of pump .I would automatically decrease 1 or the displacement. of motor? would -=automatically increase, as explained above, and -thereby-deceleratemotors? and 3l until the volumetric intake of motor" 3| exactly equalled the volumetric outputofpump 30.

A drive zconstructed aocording to the invention 5 us-ha rp ns 9 4m e ate eann a reference speed regardless of whether the reference speed is constant or variable, it is capable of operating through a wide range of speeds, and it is capable of exerting a substantially constant torque through one part of its speed range and maintaining a substantially constant power output during another part of the speed range.

The drive set forth herein is susceptible of various modifications and adaptations without departing from the scope of the invention which is hereby claimed as follows:

1. In a hydraulic drive including a pump adapted to be driven from a source of power and having a member shiftable to vary pump displacement, and a motor hydraulically connected to said pump to be energized by liquid therefrom and having a member shiftable to vary motor displacement, the combination of yielding means for urging said pump member toward and normally holding it in a position in which pump displacement is zero, yielding means for urging said motor member toward and normally holding it in a position in which motor displacement is maximum, a first servo-motor adapted when operated to move said pump member toward a position in which pump displacement is maximum, a second servo-motor adapted when operated to move said motor member in a direction to decrease motor displacement, speed control means for effecting operation of said first servo-motor, and means responsive to said pump member reaching a predetermined maximum displacement position for causing said speed control means to efiect operation of said second servo-motor.

2. In a hydraulic drive including a pump adapted to be driven from a source of power and having a member shiftable to vary pump displacement, and a motor hydraulically connected to said pump to be energized by liquid therefrom and having a member shiftable to vary motor displacement, the combination of yielding means for urging said pump member toward and normally holding it in a position in which pump displacement is zero, yielding means for urging said motor member toward and normally holding it in a position in which motor displacement is maximum, a first servo-motor adapted when operated to move said pump member toward a position in which pump displacement is maximum, a second servo-motor adapted when operated to move said motor member in a direction to decrease motor displacement, speed control means adapted when effective to efiect operation of said first servo-motor, means responsive to said pump member reaching a predetermined maximum displacement position for causing said speed control means to efiect operation of said second servomotor, and means for rendering said speed control means efiectiveor ineffective.

3. In a hydraulic drive including a pump adapted to be driven from a source of power and having a member shiftable to vary pump displacement, and a motor hydraulically connected to said pump to be energized by liquid therefrom and having a member shiftable to vary motor displacement, the combination of yielding means for urging said pump member toward and normally holdin it in a position in which pump displacement is zero, yielding means for urging said motor member toward and normally holding it in a position in which motor displacement is maximum, a first servo-motor adapted when operated to move said pump member toward a position in which pump displacement is maximum,

a. second servo-motor adapted when operated to move said motor member in a direction to decrease motor displacement, speed control means for efiecting operation of said first servo-motor, means responsive to said pump member reaching a predetermined maximum displacement position for causing said speed control means to efiect operation of said second servo-motor, and means for limiting the rate at which said servo-motors are operated.

4. In a hydraulic drive including a pump adapted to be driven from a source of power and having a member shiftable to vary pump displacement, and a motor hydraulically connected to said pump to be energized by liquid therefrom and having a member shiftable to vary motor displacement, the combination of yielding means for urging said pump member toward and normally holding it in a position in which pump displacement is zero, yielding means for urging said motor member toward and normally holding it in a position in which motor displacement is maximum, a first servo-motor adapted when operated to move said pump member toward a position in which pump displacement is maximum, a stop for preventing further movement of said pump member after it reaches a predetermined maximum displacement position, a second servo-motor adapted when operated to move said motor member in a direction to decrease motor displacement, and means for supplying pressure liquid to said servo-motors to efiect operation thereof, said servo-motors and said yielding means being so proportioned that said first servo-motor will first move said pump member to said predetermined maximum displacement position and then said second servo-motor will move said motor member toward its minimum displacement position.

5. In a hydraulic drive including a pump adapted to be driven from a source of power and having a member shiftable to vary pump displacement, and a motor hydraulically connected to said pump to be energized by liquid therefrom and having a member shiitable to vary motor displacement, the combination of yielding means for urging said pump member toward and normally holding it in a position in which pump displacement is zero, yielding means for urging said motor member toward and normally holding it in a position in which motor displacement is maximum, a first servo-motor adapted when operated to move said pump member toward a position in which pump displacement is maximum, a stop for preventing further movement of said pump member after it reaches a predetermined maximum displacement position, a second servo-motor adapted when operated to move said motor member in a direction to decrease motor displacement, means for supplying pressure liquid to said servomotors to effect operation thereof, said servomotors and said yielding means being so proportioned that said first servo-motor will first move said pump member to said predetermined maximum displacement position and then said second servo-motor will move said motor member toward its minimum displacement position, and means for limiting the rate at which liquid is supplied to said servo-motors to thereby limit the rates at which said members are moved.

6. In a hydraulic drive including a pump adapted to be driven from a source of power and having a member shiitable to vary pump displacement, and a motor hydraulically connected to said pump to be energized by liquid therefrom placement, the combination of yielding means for 9 urging said pump member toward and normally holding it in a position in which pump displacementis zero, yielding means for urging-said motor member toward and normally holding it in a position in which motor displacement is maximum, a first servo-motor "adapted when operated to move said pump member toward a position in which pump displacement'is maximum, astop for preventing further movementof said pump member after it reaches a predetermined maximum displacement position} a, second servo-motor adapted when operated to move said motor member in a direction to decrease motor displacement, means for supplying pressureliquid to said servomotors to effect operation thereof, said servomotors and said yielding means being so proportioned that said firstservo-motor will first move said pump member to said predetermined maximum displacement position. and then said second servo-motor will move said motor member toward its minimum displacement position, means for limiting the rate at which liquid is supplied to said servo-motors to thereby limit the rates at which said members are moved, and means for permitting liquid to-escape-freely from said servomotors.

'7. In 'a hydraulic drive including a main pump adapted to be driven from a source of power and having a member shiftableto vary pump displacement, anda main motor hydraulically connected to said pump to be energized by liquid therefrom and havinga member shiftable to vary motor displacement, the combination of yielding means for urging said pump member toward and normally holding it in a position in which pump displacement is zero, yielding means for urging said motor member toward and normally holding it in a position in which motor displacement is maximum, a first servo-motor adapted when operated to move said pump member toward a position in which pump displacement is maximum, a second servo-motor adapted when operated to move said motor member in a direction to decrease motor displacement, a pilot pump driven at a reference speed, a pilot motor driven in unison with said main motor, a fluid channel connecting the outlet of said pilot pump to the inlet of said pilot motor, and means responsive to said pilot pump discharging liquid in excess of the liquid that can enter said pilot motor for first directing excess liquid to said first servo-motor to cause it to move said pump member to a predetermined position and for thereafter directing additional excess liquid if any to said second servomotor to effect operation thereof.

8. In a hydraulic drive including a main pump adapted to be driven from a source of power and having a member shiftable to vary pump displacement, and a main motor hydraulically connected to said pump to be energized by liquid therefrom and having a member shiftable to vary motor displacement, the combination of yielding means for urging said pump member toward and normally holding it in a position in which pump displacement is zero, yielding means for urging said motor member toward and normally holding it in a position in which motor displacement is maximum, a first servo-motor adapted when operated to move said pump member toward a position in which pump displacement is maximum, a second servo-motor adapted when operated to move said motor member in a direction to decrease motor displacement, a pilot pump driven at a reference speed, a pilot motor driven in unison with said main motor, a fluid channel connecting the outlet of said pilot pump to the inlet of said pilot motor, and channel means connecting said fluid channel to both of saidfservomotors so that any difference between the volumetric output of said pilot pump and the volumetric input of said pilot motor causes liquid to flow to or from one or both of said servomotors to eifect operation therof, said servomotors being so proportioned relative to each other and to said yielding means that liquid delivered thereto operates said first servo-motor through a predetermined distance before it-operates said second servo-motor.

' 9. In a hydraulic drive including a main pump adapted to be driven from a source of power and having a member shiftable to vary pump displacement, and a main motor hydraulically connected to said pump to be energized by liquid therefrom and having a member shiftable to vary motor displacement, the combination of yielding means for urging said pump member toward, and normally holding it in a position in which pump displacement is zero, yielding means for urging said motor member toward and normally holding it in a position in which motor displacement is maximum, a firstservo-mo'tor adapted when operated to move said pump member toward a position in which pump displacement is maximum, a second servo-motor adapted when operated to move said motor member in a direction to decrease motor displacement, apilot pump driven at a reference speed, a pilot motor driven in unison with said main motor, a fluid channel connecting the outlet of said pilot pump to the inlet of said pilot motor, channel means connecting said fluid channel to both of said servomotors so that any difference between the volumetric output of said pilot pump and the volumetric input of said pilot motor causes liquid to flow to or from one or both of said servo-motors to effect operation thereof, said servo-motors being so proportioned relative to each other and to said yielding means that liquid delivered thereto operates said first servo-motor through a predetermined distance before it operates said second servo-motor, and a start and stop valve adapted to short circuit said pilot pump and to connect said servo-motors to exhaust.

10. In a hydraulic drive including a main pump adapted to be driven from a source of power and having a member shiftable .to vary pump displacement, and a main motor hydraulically connected to said pump to be energized by liquid therefrom and having a member shiftable to vary motor displacement, the combination of yielding means for urging said pump member toward and normally holding it in a position in which pump displacement is zero, yielding means for urging said motor member toward and. normally holding it in a position in which motor displacement is maximum, a first servomotor adapted when operated to move said pump member toward a position in which pump displacement is maximum, a second servo-motor adapted when operated to move said motor member in a direction to decrease motor displacement, a pilot pump driven at a reference s eed, a pilot motor driven in unison with said. main motor, a fluid channel connectin the outlet of said pilot pump to the inlet of said pilot motor, channel means connecting said fluid channel to both of said servo-motors so that any difference between the volumetric output of said p lot pump and the volumetric input of said pilot motor causes liquid to flow to or from one or both of said servo-motors to effect operation thereof, and a choke and a check valve connected into said channel means in parallel with each other to limit the rate of liquid flow to said servo-motors and to permit unrestricted flow therefrom, said servo-motors being so proportioned relative to each other and to said yielding means that liquid delivered thereto operates said first servo-motor through a predetermined distance before it operates said second servo-motor.

11. In a drive including a main pump having a displacement varying member shiftable between maximum and zero displacement positions, a main hydraulic motor connected in series with said pump and having a displacement varying member shiftable between maximum and minimum displacement positions, and means for continuously driving said pump, the combination of a first servo-motor for urging said motor member to its maximum displacement position, a second servo motor for urging said pump member to its zero displacement position, means for supplying each of said servo-motors with liquid at a substantially constant pressure, a third servo-motor for moving said motor member toward its minimum displacement position, a fourth servo-motor for moving said pump member toward its maximum displacement position and having an effective pressure area greater than that of said third servo-motor, means for stopping said pump member in its maximum displacement position, a pilot pump driven at a sub- 12 stantially constant speed, a pilot motor driven'in unison with said main motor, fluid channels connecting the outlet of said pilot pump to the inlet of said pilot motor and to said third and fourth servo-motors so that liquid delivered by said pilot pump in excess of that consumed by said pilot motor will flow to said fourth servo-motor and cause it to increase the displacement of said main pump to its maximum and then cause said third servo-motor to decrease the displacement of said main motor, means for adjusting the displacement of said pilot pump to thereby regulate the speed of said main motor, and a start and stop valve connected to said channels for bypassing said pilot pump.

RANSOM TYLER.

REFERENCES CITED The following references are of record in th file of this patent:

UNITED STATES PATENTS Number Name Date 1,656,544 Thoma Jan. 17, 1928 2,192,539 Condon Mar. 5, 1940 2,255,963 Benedek Sept. 16, 1941 FOREIGN PATENTS Number Country Date 1 19,452 Great Britain Aug. 31, 1911 222,647 Great Britain Oct. 9, 1924 519,686 Germany Mar. 3, 1931